In geographical areas of the earth where the production of subterranean hydrocarbons has become difficult due to the nature of the oil or natural subterranean conditions, the use of stimulating methods is well known in the industry. One such method identified more particularly as steam injection or steam drive, relates to a method wherein steam is injected directly into the hydrocarbon containing segment of the substrate.
The injected steam is of necessity at a sufficient pressure to overcome the natural pressure which would ordinarily be exerted on the contained hydrocarbon in liquid or gaseous form. The stimulating steam injected into the reservoir function to drive the hydrocarbon toward an area of lower pressure such as a production well or wells positioned at a distance from the injection well.
In principle, the steam stimulating method utilizes steam which is usually generated at a remote point and is delivered by piping to the respective well heads of the injection wells.
While the steam injection procedure is familiar to the industry, the degree of efficiency with which it operates is dependent to a large degree on the quality and the flow rate of the steam being injected. It may also depend on the capability of the substrate to be heated, and on the contained hydrocarbon to flow toward one or more of the production wells.
The prior teachers that one presently practiced method for controlling steam injection rate to a well or wells, is through use of a critical flow choke bean. This apparatus is positioned immediately upstream of the well head and operates at a critical flow condition. Thus, the steam's mass flow rate depends on the steam supply pressure and on the size of the choke bean. For a given steam pressure, the resulting flow rate will in effect depend on the size of the choke bean.
One detriment to the use of such a flow rate control device is the excessive pressure drop required across the choke bean to achieve a critical flow condition. This pressure drop is found in practice to be approximately 45% to 55% of the steam pressure upstream of the bean. Stated otherwise, steam pressure immediately downstream of the choke bean will be only 45% to 55% of the choke inlet steam pressure. Thus, if reservoir pressure is relatively high, or the steam supply pressure isn't high enough, the choke bean serves only as a flow restriction and cannot function to determine flow rate.
In summary, a primary fault endemic to the use of any choke bean as a flow rate control device in the steam injection or steam stimulating process, is the severe decrease in steam pressure realized across the bean. Operationally, when a relatively high reservoir pressure is encountered, the steam upstream pressure must be commensurably increased. However, such an increase in steam pressure will result in an increase in production costs associated with steam generation and distribution.